Abiotic stress is the primary cause of crop loss worldwide, causing average yield losses more than 50% for major crops (Boyer, J. S. (1982) Science 218:443-448; Bray, E. A. et al. (2000) In Biochemistry and Molecular Biology of Plants, edited by Buchannan, B. B. et al., Amer. Soc. Plant Biol., pp. 1158-1249). Plants are sessile and have to adjust to the prevailing environmental conditions of their surroundings. This has led to their development of a great plasticity in gene regulation, morphogenesis, and metabolism. Adaption and defense strategies involve the activation of genes encoding proteins important in the acclimation or defense towards the different stresses.
Drought is one of the major abiotic stresses that limit crop productivity worldwide, and exposure of plants to a water-limiting environment during various developmental stages appears to activate various physiological and developmental changes. Although many reviews on molecular mechanisms of abiotic stress responses and genetic regulatory networks of drought stress tolerance have been published (Valliyodan, B., and Nguyen, H. T. (2006) Curr. Opin. Plant Biol. 9:189-195; Wang, W., et al. (2003) Planta 218:1-14; Vinocur, B., and Altman, A. (2005) Curr. Opin. Biotechnol. 16:123-132; Chaves, M. M., and Oliveira, M. M. (2004) J. Exp. Bot. 55:2365-2384; Shinozaki, K., et al. (2003) Curr. Opin. Plant Biol. 6:410-417; Yamaguchi-Shinozaki, K., and Shinozaki, K. (2005) Trends Plant Sci. 10:88-94), it remains a major challenge in biology to understand the basic biochemical and molecular mechanisms for drought stress perception, transduction and tolerance. Genetic research has shown that drought tolerance is a quantitative trait, controlled by many genes. Molecular marker-assisted breeding has led to improved drought tolerance in crops. However, marker accuracy and breeding efficiency remain problematic (Ashraf M. (2010) Biotechnol. Adv. 28:169-183). Transgenic approaches to engineering drought tolerance in crops have made progress (Vinocur B. and Altman A. (2005) Curr. Opin. Biotechnol. 16:123-132; Lawlor D W. (2013) J. Exp. Bot. 64:83-108).
The absorption of nitrogen by plants plays an important role in their growth (Gallais et al. (2004), J. Exp. Bot. 55(396):295-306). Plants synthesize amino acids from inorganic nitrogen absorbed from environment, so, nitrogen fertilization has been a powerful tool for increasing yield of cultivated plants, such as maize. Lack of sufficient plant-available nitrogen for optimum growth and development may be considered as an abiotic stress. In order to avoid pollution by nitrates and to maintain a sufficient profit margin, today farmers desire to reduce the use of nitrogen fertilizer. If a plant variety has increased nitrogen assimilation capacity, it would also be expected to have increased growth and yield. In summary, plant varieties that have better nitrogen use efficiency (NUE) are desirable.
Glutamate receptors (GLR) can bind glutamate and function through ligand gated ion channels or G-protein coupled receptors in the major excitatory neurotransmitter in mammals. GLR-like genes have been identified in plants (Davenport R. (2002) Annals of Botany 90:549-557).